[FFmpeg-user] Decimation with ppsrc=1 malfunctions

Mark Filipak (ffmpeg) markfilipak at bog.us
Sun Dec 27 19:42:32 EET 2020

On 12/27/2020 06:19 AM, Paul B Mahol wrote:
> On Sun, Dec 27, 2020 at 5:00 AM Mark Filipak (ffmpeg) <markfilipak at bog.us>
> wrote:
>> Decimation with ppsrc=1 malfunctions.

>> Paul fixed this somewhat, but it's still in trouble.
>> Since I eliminated threading and decimation as the cause, I suspect that
>> frames numbers are being
>> bollixed.
>> The report package is here:
>> https://www.dropbox.com/t/zu4cezneCJIsdmUu

> decimate filter only drops frames, it never fixes combing.

Of course.

> Do you know exact telecine pattern used in your samples?

Look at 'source.mkv frames 1018-1029.png'. I have a scenario that explains why a sober engineer 
would be forced to do what was done, but what's the point? Combing is not the issue. The issue is 
that the duplicate (modulo 5) frames found in preprocessed.mkv are not being removed from source.mkv.

source.mkv & preprocessed.mkv are 172 frames. target.mkv should be 138 frames. It is 138 frames, but 
they are the wrong frames. Worse, the frames removed are not modulo 5 but vary instead. The only 
cause I could imagine was a mixup due to a threading mixup but single-threading didn't solve the 

Over to you...  :-)

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